Synthesis and characterization of 2-(anthracene-9-yl)-4,5-diphenyl-1H-imidazole derivatives as environmentally sensitive fluorophores

2-(Anthracene-9-yl)-4,5-diphenyl-1H-imidazole (ADPI) provides an intriguing molecular platform for developing organic fluorophores with diverse properties and fluorescence performances. However, derivatives of ADPI have not yet been well explored and extensive studies are warranted. To shed more light on this, we have synthesized a series of π-extended ADPIs through a concise synthetic route involving an efficient cross-condensation reaction followed by Pd-catalyzed Suzuki cross-coupling. The obtained compounds were subjected to X-ray single crystallographic analysis to understand their molecular conformational and solid-state packing properties. Furthermore, UV-Vis absorption and fluorescence spectroscopic analyses were conducted. Our experimental results have disclosed interesting solvatofluorochromic properties of these compounds which are useful for solvent polarity-sensitive applications. The presence of an amphoteric imidazolyl group in the ADPI derivatives also renders them sensitive fluorescence responses to strong protic acids (e.g., trifluoroacetic acid) as well as fluoride anion. It transpires that the fluorescence changes are dependent on the functional groups attached to the ADPI core, suggesting a bottom-up molecular tuning approach for development of fluorophores and chemosensors with diverse functions.


Introduction
Imidazole is an important ve-membered heterocycle that is widely present in natural products, 1 biomolecules, 2 pharmaceutical drugs, [3][4][5][6] synthetic polymers, 7 selective ligands, 8 functional organic chromophores, 9,10 and to just name a few.Over the past decade, planar imidazole-fused polycyclic aromatic systems, such as phenanthroimidazole [11][12][13][14][15] and pyrenoimidazole derivatives, [16][17][18][19] have captured considerable attention in the eld of organic optoelectronics primarily because of their excellent electroluminescence performances and thermal stability.Changing their fused p-motif into nonfused polyaryl-substitution results in increased steric clashing among the aryl groups and force the molecular structures to take more twisted conformations (such as that illustrated in Fig. 1).This type of variation has been found to be benecial for bringing about new optoelectronic performances that are very different from their planar counterparts.For example, Alreja and Kaur recently reported that 2-(anthracene-9-yl)-4,5diphenyl-1H-imidazole (ADPI, see Fig. 1) shows more selective colorimetric and uorescence sensing properties for certain transition metal ions (e.g., Cu 2+ ) than an analogous uorophore that contains a planar imidazole-fused phenanthroline. 20Li et al. in 2014 discovered that ADPI exhibits intriguing polymorphism-dependent uorescence and piezochromic behavior in the solid state. 21Pan and co-workers reported that ADPI molecules are packed in a non-parallel fashion in the crystalline state, yielding single-crystalline microwires that can act as efficient optical waveguides. 22esides the above-mentioned features, ADPI and its numerous analogues have also been investigated in terms of Fig. 1 The design strategy for derivatizing the p-extended ADPI (p-ADPI) system in this work.
optical absorption/emission properties, 23 solvatouorochromism, 24 electroluminescence, 25 and catalytic effects on Pdcatalyzed coupling reactions 26 over the past decade.Nonetheless, the number of ADPI derivatives so far documented in the literature is still much less than many other imidazole-based pconjugated systems.Continued synthesis and characterizations of new variants of ADPI are therefore warranted.In this work, we designed a series of p-ADPIs that carry various aryl substituents at the para-positions of the ADPI's two phenyl groups (see Fig. 1).Synthetically, these p-ADPIs can be exibly prepared through efficient transition metal-catalyzed cross-coupling reactions, especially the well-established Suzuki-Miyaura cross coupling. 27Fundamentally, it is important to acquire understanding about how the aryl substituents inuence the structural, electronic, and photophysical properties.Disclosure of relevant substituent-property correlations will offer useful guidance to further exploration of ADPI-based materials in optoelectronic applications.The following presents our synthesis and systematic characterizations of four p-ADPI derivatives based on single-crystal X-ray diffraction (SCXRD), NMR, UV-Vis absorption, and uorescence analyses.

Synthesis
2-(Anthracene-9-yl)-4,5-bis(4-bromophenyl)-1H-imidazole 3 was synthesized as a key intermediate for making diverse p-ADPIs (see Fig. 2).This imidazole intermediate was readily prepared through a one-pot condensation reaction between dione 1 and anthracene-9-carbaldehyde (2) in the presence of ammonium acetate and acetic acid, the reaction mechanism of which follows the multi-component Debus-Radziszewski imidazole synthesis. 28,29Upon reuxing at 120 °C for 12 hours, this condensation reaction afforded intermediate 3 in a high yield of 86%.The two bromo groups in 3 provide synthetic handles for transition metal-mediated cross-coupling reactions.In our work, Suzuki-Miyaura cross-coupling reactions were respectively conducted between 3 and four areneboronic acids (4a-d).The reaction conditions of the Suzuki reactions involved Pd(PPh 3 ) 4 as the catalyst, cesium carbonate as the base, and a mixture of THF and water as the solvent.All the coupling reactions went smoothly at 70 °C and accomplished within 6 hours.The resulting cross-coupled products 5a-d were then obtained aer standard column chromatographic purication in satisfactory yields ranging from 60% to 76%.1][32][33] In our cases here, the crosscoupling efficiency turned out to be really good, presumably due to steric effects delivered by the 9-anthryl group that is adjacent to the imidazolyl unit of 3. Imidazole is a versatile group that can act as both hydrogen bond donor and acceptor.For the purpose of comparative analysis in our structural and photophysical studies, we also prepared compound 6 which is an Nmethylated derivative of ADPI 3. The synthesis of 6 was done through a substitution reaction by deprotonation of 3 with sodium hydride, followed by treatment with methyl iodide.The structures of all the prepared ADPI and p-ADPI compounds were elucidated by 1 H NMR, 13 C NMR, infrared (IR) spectroscopic, and high resolution mass spectrometric (HR-MS) analyses.Detailed characterization data are provided in the ESI.†

X-ray single crystallographic properties
Single crystals of compounds 3, 5b, and 6 were successfully grown through slowly diffusing hexane into a CH 2 Cl 2 solution of 3 or letting the solutions of 5b and 6 in CH 2 Cl 2 /THF (1 : 2, v/v) slowly evaporate at room temperature.The crystal structures of these compounds were then elucidated by XRD analysis to understand their molecular structural and solid-state packing properties.Fig. 3A illustrates one of the molecular structures in the unit cell of the single crystal of compound 3.The crystal structure of 3 presents a triclinic system with P-1 space group.As expected, the arene groups around the imidazole core in 3 all  rotate signicantly to avoid steric clashing.The imidazolyl group of 3 acts as both a hydrogen bond donor and a hydrogen bond acceptor, affording a network of hydrogen bonded assembly in the crystal structure as depicted by Fig. 3B.Along the hydrogen-bonded network, three similar hydrogen bonding interactions can be observed with N/H distances of 2.08, 2.14, and 2.01 Å, respectively.In the meantime, the intermolecular contact is further enhanced through p-stacking of anthryl rings.
The molecular structure of compound 6 is similar to that of 3 (see Fig. 4A).The presence of an N-methyl group makes the anthryl unit of 6 take a nearly perpendicular orientation with respect to the central imidazole ring at a torsion angle of 84°.The other two phenyl rings are somewhat less rotated, particularly the ring at the 4-position of imidazole shows a torsion angle of 20 to 23°with respect to the imidazole plane.The solid-state packing motif of 6, however, is very different from that of 5a.The crystal structure of 6 is in an orthorhombic system with Pna2 1 space group.Because of N-methylation, the imidazole unit of 6 lacks the ability to form intermolecular hydrogen bonding interactions.The twisted conformation of 6 thus leads to relatively loose packing in the solid state, where the dominant intermolecular forces are C-H/p stacking as shown in Fig. 4B.
The X-ray determined molecular structure of 5b is shown in Fig. 5A.Like the other two compounds, the anthryl unit in 5b takes a perpendicular orientation relative to the central imidazole ring.The two phenyl rings show torsion angles of 35-39°with respective to the imidazole unit, and 36-40°to the two benzo[b]thiophene units, respectively.The steric bulkiness of the arene groups surrounding the imidazole ring of 5b hinders the formation of intermolecular hydrogen bonds among the molecules of 5b in the crystalline state.In the crystal structure, molecules of 5b are packed with a triclinic unit cell and P-1 space group.As shown in Fig. 5B, the packing motif of 5b has no signicant p-stacking involved, but C-H/p interactions between benzo[b]thiophene units present notably.
Diffusion of methanol into a THF solution of 5b produced solvates, the solid-state structure of which was determined by Xray analysis.Fig. 6 shows the molecular structure of 5b where the imidazole unit interacts with two molecules of methanol through hydrogen bonds.Compared with the molecular conformation observed in the single crystal of 5b, the incorporation of methanol molecules in the crystal lattice causes the anthryl group to take a relatively small torsion angle of 57°with respect to the imidazole ring.It is also interesting to observe that the molecular conformation of 5b in the solvate structure possesses a C 2 symmetry.As such, the solvate of 5b shows an intimate packing motif in the solid state, forming a monoclinic I2/a system.In the crystal packing, methanol molecules lled in the space between the molecules of 5b through hydrogen bonds

UV-Vis absorption and uorescence spectroscopic properties
The electronic absorption properties of compounds 3, 5a-d, and 6 were investigated by UV-Vis absorption spectroscopic analysis.
As shown in Fig. 7A, the spectra of ADPIs 3 and 6 measured in CH 2 Cl 2 show three vibronic bands at 388, 368, and 348 nm in the low-energy region, which are characteristic of the p / p* transitions at the anthryl unit.Similar absorption bands are discernible in the spectra of 5a-d in CH 2 Cl 2 (highlighted by dashed lines in Fig. 7A), but they signicantly overlap with the absorption bands of the arene groups appended to the ADPI scaffold.Detailed UV-Vis absorption data are summarized in Table 1.Fig. 7B shows the normalized uorescence spectra of 3, 5a-d, and 6 measured in CH 2 Cl 2 .As can be seen, all the emission spectra exhibit a smooth Gaussian-type prole with different maximum emission wavelengths (l em ) ranging from 469 to 522 nm (see Table 1 for details).These emission peaks are considerably redshied than pristine anthracene, which typically shows a set of vibronic bands in the spectral region of 360-460 nm. 34According to the UV-Vis analysis, the excitation light should be dominantly absorbed by the anthracene unit, promoting it to the rst excited state (S 1 ) through vertical electronic transition.
It is interesting to note that the l em of 6 is blueshied by 8 nm relative to 3, although they possess the same p-frameworks except that 6 is methylated at the imidazolyl unit and 3 has a free imidazole ring.Since both of them show nearly identical low-energy absorption bands, the difference in their maximum emission wavelengths can be explained by that the anthracene and imidazole rings of 6 in the rst excited state are more twisted than that of 3 due to the N-methyl group of 6.For p-ADPIs 5a-d, the maximum emission wavelengths are further redshied; in particular, the effects of strong electron-donating diphenylamine and carbazole groups in 5c and 5d are more signicant than the others.
In a previous study by Li and co-workers, 21 the uorescence of ADPIs was reported to exhibit solvatouorochormic effects.In view of these properties, we also investigated the uorescence behavior of our synthesized ADPIs and p-ADPIs in organic solvents with different degrees of polarity.Fig. 8 shows the correlations of the maximum emission energies of 3, 5a-d, and 6 with the polarity indexes (P 0 ) of various solvents.For all of the six ADPI derivatives, the correlation plots exhibit a general trend of decreasing emission energy (i.e., redshied l em ) with increasing solvent polarity, but signicant irregularities can be seen in the polarity range of 3-6; particularly, methanol and ethanol are the most notable outliers.The solvent-dependent uorescence suggests that the emissive states of these compounds possesse intramolecular charge-transfer (ICT) character.As demonstrated in the X-ray analysis of 5b, protic solvents (methanol and ethanol) can form hydrogen bonding interactions with the imidazole C]N group and hence contribute more stabilization to the ground state.This effect in turn modies the energy gap between the emissive state and the ground state to signicantly affect the emission energy.
To better understand the solvatouorochromic effects observed for the ADPIs and p-ADPIs, density functional theory (DFT) calculations were carried out on compound 3 and its frontier molecular orbital properties are illustrated in Fig. 9A.The highest occupied molecular orbital (HOMO) is distributed among the anthracene and imidazole units, while the lowest unoccupied molecular orbital (LUMO) is predominantly located in the anthracene unit.When an ADPI (e.g., 3) is photoexcited, the vertical electronic transition contains a character of mainly HOMO / LUMO transition.From the FMO analysis, it is reasonable to say that the S 0 / S 1 transition  should result in an ICT from imidazole to anthracene.Moreover, the S 1 is more polar in nature than the S 0 due to the occurrence of ICT.As such, an aprotic dipolar solvent should provide more stabilization on the S 1 than the S 0 state to reduce the energy gap and cause redshied l em .On the other hand, if the solvent is a hydrogen bond donor (e.g., an alcohol), the ground-state ADPI can form signicant hydrogen bonding interactions similar to the case observed in the methanolsolvated X-ray structure of 5b.These interactions, contrary to the polarity effect, stabilize the ground (S 0 ) state more than the rst excited (S 1 ) state.In this case, the energy gap between S 0 and S 1 is widened and the l em is blueshied.Overall, the observed solvatouorochromic effects of 3, 5a-d, and 6 point toward a potential of utilizing these ADPI derivatives as a class of environment-sensitive uorophores for probing different solvent properties.

Interactions of ADPI derivatives with triuoroacetic acid
The imidazole group has an amphoteric character, allowing it to act as both an acid and a base. 8,35In view of this property, we subsequently investigated our synthesized ADPI derivatives in terms of their spectral responses to interactions with acids.In our experiments, a strong organic acid, triuoroacetic acid (TFA), was used to protonate compounds 3, 5a-d, and 6, respectively, in organic solution, and the detailed processes were monitored by UV-Vis, uorescence, and NMR analysis.Fig. 10 shows the results of titration of ADPI derivatives 3 and 6 with TFA.Interaction of compound 3 with TFA caused the three characteristic anthracene absorption bands in the lowenergy region (350-390 nm) to be slightly redshied by ca. 5 nm (see Fig. 10A).In the high-energy region of the absorption spectrum, two isosbestic points can be clearly seen at 303 and 270 nm, respectively.In contrast to the moderate changes observed in the UV-Vis titration, the uorescence prole of 3 was found to be substantially quenched by TFA titration.As shown in Fig. 10C, the uorescence intensity of 3 at the maximum emission wavelength is quenched by nearly 93% aer interacting with 8.65 mole equiv. of TFA.The uorescence quenching trend can be well described by a logistic nonlinear regression model (see the inset of Fig. 10C).
The UV-Vis absorption spectrum of N-methylated 6 varies in a similar way to that of 3 during the TFA titration (Fig. 10B), but the uorescence of 6 responds to TFA titration quite differently.As shown in Fig. 10D, the uorescence intensity of 6 increases linearly with the amount of added TFA from 0 to 5.34 mole equiv., along with a noticeable blueshi of the maximum emission wavelength.Further increase in TFA addition results in a trend of uorescence quenching that follows a non-linear   Paper RSC Advances logistic model (see Fig. 10D).At the saturation point, the uorescence of 6 is only quenched by ca.20%, which is signicantly different from the nearly quantitative quenching effect observed for ADPI 3.
The uorescence titration data indicates that the interactions of 6 with TFA undergo two separate stages.To better understand the protonation processes involved, 1 H NMR titration of 6 with TFA was performed in deuterated DMSO.Fig. 11 shows the NMR titration results, where characteristic signals due to certain protons on the anthracene units (labelled as Ha-Hc) are highlighted to show the trend of spectral changes.When the amount of TFA added is 0.25 mole equiv., the aromatic proton signals are complex and broad, indicating a dynamic process of proton exchange on the imidazolyl unit.At this stage, it is possible that the presence of TFA induces hydrogen bonding interactions with 6, making the p-framework of 6 more twisted and less rotatable.The enhanced and blueshied uorescence observed in the early stage of uorescence titration are consistent with this argument.When the amount of TFA is more than 0.50 mole equiv., the aromatic signals of 6 show better-resolved patterns in which the central proton (Ha) of the anthracene unit becomes signicantly downeld-shied to 9.11 ppm.The other anthryl protons are also downeld-shied but in a less magnitude.This can be explained by that the imidazole is protonated on the C]N site, converting it into a more electron-withdrawing imidazolium ring.As a result of protonation, the excited state of 6 can be deactivated more efficiently through some non-radiative decay pathways to cause uorescence quenching; for example, the photoinduced electron transfer from anthracene (donor) to imidazolium ion (acceptor). 36n contrast to N-methylated ADPI 6, TFA titrations of ADPI 3 and its p-extended derivatives 5a-d all show similar uorescence quenching behavior.As can be seen from Fig. 12, the uorescence intensities of p-ADPIs 5a-b and 5d are nearly quantitatively quenched at the saturation point, which is similar to the results of ADPI 3. Obviously, the imidazolyl N-H group in these compounds plays a key role in the acid-induced quenching effect.It is interesting to note that 1 H NMR monitoring the titration of 5a with TFA reveals three stages of change (see Fig. 13), which is different from the one-step protonation of ADPI 3 (see Fig. S-12 in the ESI †).It is likely that the anisolyl groups attached to compound 5a also participate in interactions with TFA and hence result in strong uorescence quenching.In the case of 5c where diphenylamino groups are appendages, the uorescence is quenched by ca.70% at the saturation point.The exact reason for compound 5c to retain a relatively high level of uorescence during TFA titration is not quite clear.However, it is tentatively proposed that protonation of the diphenylamino groups makes the structure of 5c more twisted and sterically hindered.This effect somewhat decelerates the non-radiative decays of the excited state of 5c.

Interactions of ADPI derivatives with uoride anion
The performance of free imidazole as a hydrogen bond donor 7,37 inspired us to investigate the potential of ADPIs 3 and 5a-d as uoride anion receptors and/or sensors, considering the strong hydrogen bond acceptor ability of uoride anion. 38,39Fig. 14 shows the UV-Vis and uorescence titration results of ADPI 3 with tetrabutylammonium uoride (TABF) in CH 2 Cl 2 .As can be seen from the UV-Vis data (Fig. 14A), a broad long-wavelength absorption band centering at ca. 450 nm emerges with increasing addition of TBAF to the solution of 3.This band can be attributed to the deprotonation of the imidazolyl N-H group by uoride anion, 38 which leads to enhanced p-electron delocalization and reduced HOMO-LUMO gap.In the meantime,  the high-energy absorption bands due to anthracene in the range of 340-390 nm are also observed to grow in intensity with increasing titration of TBAF.The change in UV-Vis absorbance at 450 nm follows a nonlinear model using the Hill equation 40 (see the plot in the inset of Fig. 14A).
The uorescence titration results of 3 with TBAF are provided in Fig. 14B.During the titration, the emission intensity of 3 gradually deacreases with increasing amount of TBAF, showing a straightforward uorescence quenching effect.The trend of uorescence quenching at the maximum emission wavelength of 3 follows a nonlinear logistic model very well, and at the saturation point of titration the uorescence is quenched by more than 80%.Moreover, the uorescence spectral proles show a signicant redshi as the titration progresses.[43] To further understand the interactions 3 with uoride anion, 1 H NMR titration experiments were carried out and the results are illustrated in Fig. 15.It is interesting to note that when only a small amount of TBAF (0.2-0.6 mole equiv.) was added to the solution of 3, the NMR signals appeared to be signicantly broadened.This stage of NMR responses can be attributed to a rapid exchange between free ADPI 3 and its hydrogen bonded complex, [3/F − ].When the amount of TBAF was increased to more than 0.8 mole equiv., the NMR spetral prole showed wellresolved features, indicating another stage of interaction with uoride anion.Given the basicity of uoride anion, it is believed that the imidazolyl unit is deprotonated at the N-H site to yield an imidazole anion.It is worth noting that in the NMR spectrum, the central anthryl proton that is labelled as Ha gives a singlet at 8.82 ppm, while the other two anthryl protons labelled as Hb and Hc appear as two pseudo doublets at 8.21 and 7.93 ppm, respectively.During the titration, these peaks are considerably shied.The singlet Ha and the proton adjacent to it, which is labelled as Hb, are both shied to the upeld (8.39 and 8.00 ppm, respectively).The proton (labelled as Hc) that is close to the imidazole unit, however, is considerably downeld-shied to 8.83 ppm.It has been reported that imidazole anion has a greater degree of aromaticity than neutral imidazole ring. 44,45Upon uoride titration, the anthryl proton Hc of 3 is therefore subjected to increasing deshielding effect of imidazole anion ring current, which in turn causes its resonance frequency to be substantially downeld-shied.The results of UV-Vis and uorescence titrations of p-ADPIs 5a and 5d with TBAF are shown in Fig. 16.Similar to compound 3, both 5a and 5d show a notable growth of a long-wavelength broad absorption band in their UV-Vis spectra, ranging from ca. 440 to 550 nm.The uorescence spectra of 5a and 5d exhibit signicant quenching in response to the titration of TBAF.For compound 5a, the uorescence is almost quantitatively quenched at the saturation point of titration.Compound 5d shows uorescence responses like those of 3; that is, the emission peak is redshied signicantly at the end of titration (see Fig. 16D).
As can be seen from Fig. 17A and B, the UV-Vis spectra of 5b and 5c exhibit similar behaviors to the other ADPI derivatives during TBAF titrations.The uorescence spectra of 5b and 5c, however, show very different trends at the early stage of titration.Interestingly, both compounds exhibit uorescence enhancement when they interact with a relatively small amount of TBAF.Especially, the uorescence of compound 5c is increased by more than one fold when interacting with ca.1.5 mole equiv. of TBAF.As the titration continues, the uorescence behavior changes to a trend of quenching that is similar to the other ADPI derivatives.It is also interesting to comment that the maximum emission band of 5c at the end of titration is notably blueshied relative to that of pristine 5c.Clearly, the electron-donating diphenylamino groups in 5c are responsible for this unusual spectral behavior.It is possible that the electron pushing between the diphenylamino and anionic imidazole ring causes the molecular framework to be more twisted and less p-conjugated.

Conclusions
We herein report a systematic study of a series of ADPI derivatives that contain p-extended units and appendage groups with different electronic effects.These compounds can be readily accessed through concise synthetic routes with satisfactory to good yields.X-ray single crystallographic analysis shows that the ADPI compounds take twisted molecular conformations and are packed in very different motifs depending on the appendage groups attached.All of these compounds show signicant solvatouorochromic effects that can be potentially useful as molecular probes for solvent polarity.The amphoteric properties of the imidazole unit in these compounds allow them to interact with strong protic acids (e.g., TFA) and hard anionic species (e.g., uoride anion).Signicant UV-Vis and uorescence responses to TFA and uoride anion have been thoroughly examined and analyzed.Our results point to the applicability of p-ADPIs in achieving colorimetric and uorescence sensing of various acidic and/or anionic species.Moreover, the observations of some ADPI derivatives showing unexpected uorescence enhancement in response to TFA or TBAF titrations indicate that the appendage groups in these compounds play an important role in dictating their photophysical properties.It is therefore anticipated that preparation of more arene-appended p-ADPIs would allow diversely behaving uorophores to be attained, which may be further applied to form sensor arrays for rapid and accurate detection of chemical species of interest.Studies along this direction are underway.
Fig. 17 UV-Vis spectra monitoring the titrations of (A) 5b (1.17 × 10 −4 M in CH 2 Cl 2 ) with TBAF (0.00 to 50.99 mole equiv.)and (B) 5c (9.06 × 10 −5 M in CH 2 Cl 2 ) with TBAF (0.00 to 44.59 mole equiv.).Insets: plot of (A − A 0 ) against the mole equiv. of TBAF, where A 0 and A are the absorbance values measured at 500 nm before and during titration of TBAF.Fluorescence spectra monitoring the titrations of (C) 5b (1.17 × 10 −4 M in CH 2 Cl 2 ) with TBAF (0.00 to 50.99 mole equiv.)and (D) 5c (9.06 × 10 −4 M in CH 2 Cl 2 ) with TBAF (0.00 to 62.17 mole equiv.).Insets: plot of (F 0 − F)/F 0 vs. the mole equiv. of TFA, where F 0 is the fluorescence intensity at the maximum emission wavelength before titration and F is the fluorescence intensity at the same wavelength during titration.Arrows indicate the trends of spectral changes with increasing addition of TBAF.

Fig. 3 (
Fig. 3 (A) ORTEP drawing (at 50% ellipsoid probability) of the molecular structure of 3 determined by single-crystal X-ray diffraction analysis.(B) Hydrogen bonding interactions in the crystal structure of 3 with hydrogen bond distances highlighted in Å. CCDC 2354158.

Fig. 5 (
Fig. 5 (A) ORTEP drawing (at 50% ellipsoid probability) of the molecular structure of 5b determined by single-crystal X-ray diffraction analysis.(B) Packing motif shown in the crystal structure of 5b.CCDC 2354160.

Fig. 6 (
Fig. 6 (A) ORTEP drawing (at 50% ellipsoid probability) of the molecular structure of 5b solvate determined by single-crystal X-ray diffraction analysis.(B) Packing motif shown in the crystal structure of 5b solvate.CCDC 2354150.

Fig. 4 (
Fig. 4 (A) ORTEP drawing (at 50% ellipsoid probability) of the molecular structure of 6 determined by single-crystal X-ray diffraction analysis.(B) Packing motif shown in the crystal structure of 6. CCDC 2354159.

Fig. 14 (
Fig.14 (A) UV-Vis spectra monitoring the titration of 3 (9.51× 10 −5 M in CH 2 Cl 2 ) with TBAF (0.00 to 50.79 mole equiv.).Inset: plot of (A − A 0 ) against the mole equiv. of TBAF, where A 0 and A are the absorbance values measured at 450 nm before and during titration of TBAF.(B) Fluorescence spectra monitoring the titration of 3 (9.51× 10 −5 M in CH 2 Cl 2 ) with TBAF (0.00 to 43.86 mole equiv.).Inset: plot of (F 0 − F)/F 0 vs. the mole equiv. of TFA, where F 0 is the fluorescence intensity at the maximum emission wavelength before titration and F is the fluorescence intensity at the same wavelength during titration.Arrows indicate the trends of spectral changes with increasing addition of TBAF.

Fig. 16
Fig.16UV-Vis spectra monitoring the titrations of (A) 5a (8.62 × 10 −5 M in CH 2 Cl 2 ) with TBAF (0.00 to 33.26 mole equiv.)and (B) 5d (8.25 × 10 −5 M in CH 2 Cl 2 ) with TBAF (0.00 to 11.63 mole equiv.).Insets: plots of (A − A 0 ) against the mole equiv. of TBAF, where A 0 and A are the absorbance values measured at 470 nm and before and during titration of TBAF.Fluorescence spectra monitoring the titrations of (C) 5a (8.62 × 10 −5 M in CH 2 Cl 2 ) with TBAF (0.00 to 43.86 mole equiv.)and (D) 5d (8.25 × 10 −5 M in CH 2 Cl 2 ) with TBAF (0.00 to 11.56 mole equiv.).Insets: plot of (F 0 − F)/F 0 vs. the mole equiv. of TFA, where F 0 is the fluorescence intensity at the maximum emission wavelength before titration and F is the fluorescence intensity at the same wavelength during titration.Arrows indicate the trends of spectral changes with increasing addition of TBAF.

Table 1
Summary of photophysical data of compounds 3, 5a-d, and 6. l abs : wavelength of UV-Vis absorption peak; 3: extinction coefficient; l em : wavelength of maximum emission; F: fluorescence quantum yield; n: Stokes shift